Solvent extraction process for producing low-nitrate and large-crystal-size puo2 sols

ABSTRACT

LOW-NITRATE PLUTONIA SOLS HAVING A NO3/PU MOLE RATIO IN THE RANGE 0.1 TO 0.4 WITH AVERAGE CRYSTALLITE DIAMETERS OF 30 TO 80 A. CAN BE PRODUCED WHEN A SOL IS PREPARED BY SOLVENT EXTRACTION OF A PLUTONIUM NITRATE SEEDED WITH A PLUTONIA SOL. WHEN THE SEEDED SOL IS TAKEN TO DRYNESS AND HEATED FOR 10 TO 120 MINUTES AT A TEMPERATURE IN THE RANGE 190-230* C., NITRATE REMOVAL OCCURS WITH CONCOMITANT CRYSTALLITE GROWTH.

SOLVENT EXTRACTION PRGCESS FOR PRODUG ENG LOW-NKTRATE ANDLARGE-CRYSTAL-SHZE Pu SOLS Milton H. Lloyd and Gthar K. Tallent, SaltRidge, and Rex E. Leuze, Lenoir City, Tenn., assignors to the UnitedStates of America as represented by the United States Atomic EnergyCommission No Drawing. Filed June 2, 1971, Ser. No. 149,357

lint. Cl. (101g 56/00 US. Cl. 25230l.1 S 7 Claims ABSTRACT OF THEDISCLOSURE Low-nitrate plutonia sols having a NO /Pu mole ratio in therange 0.1 to 0.4 with average crystallite diameters of 30 to 80 A. canbe produced when a sol is prepared by solvent extraction of a plutoniumnitrate seeded with a plutonia sol. When the seeded sol is taken todryness and heated for 10 to 120 minutes at a temperature in the range190-230 C., nitrate removal occurs with concomitant crystallite growth.

BACKGROUND OF THE INVENTION The invention described herein was made inthe course of, or under, a contract with the US. Atomic EnergyCommission.

The present invention relates to plutonia sols. More particularly, itrelates to a process for producing stable plutonia sols alone or as amixed sol such as a plutoniaurania sol. Sols of the kind referred to inthis invention are eminently useful as feed for the production ofactinide oxide dense microspheres.

In the context of sol-gel technology for the production of oxidemicrospheres, a stable sol is one which has a useful shelf life andwhich is long enough to enable the sol to be processed to a microsphere.A sol with a useful shelf life means one which maintains a sol conditionwithout settling out of the solid phase such as by precipitation or bygelling or which does not undergo any chemical change. Shelf life isimportant for production logistics, since it is generally desirable tomaintain an inventory of sol available for processing into microspheres.To speak of a stable sol in terms of shelf life alone, however, does notnecessarily qualify the sol as stable since experience has shown thatsols with long shelf life are not necessarily sufficiently operationallystable when they undergo processing to form microspheres, particularlywhere microsphere production involves mixed sols such as plutonia andurania.

The plutonia sols produced by this invention are derived principallyfrom aqueous solutions of plutonium nitrate by a solvent extractionprocess known as the Allcohol-Plutonium Nitrate Extraction process andhereinafter referred to as the APEX process. The APEX process requiresseveral steps, the first being extraction of nitric acid with an aqueousimmiscible alcohol, such as n-hexanol, to decrease thenitrate-to-plutonium mole ratio to about 2.5. The solution is thendigested at elevated temperature, about 100 C., to hydrolyze theplutonium (which is thought to exist, by then in polymeric form), thusreleasing more nitric acid. After cooling to room temperature, thesolution or sol is further extracted with n-hexanol to produce sol witha nitrate-to-plutonium mole ratio of about 1.0. Alternately, the sol canbe prepared by continuous extraction which avoids the digestion step,and this is the preferred method. Additional nitrate can be extracted toprovide a final NO /Pu mole ratio of 0.6-

0.8 in a sol. Such a sol can be concentrated to more than 1 M inplutonium where the desired plutonium concentration is reached byevaporation. This procedure is referred to by us as the regular APEXprocess. It produces a stable, highly crystalline sol, where stabilitymeans a useful shelf life. Regular APEX sols are called high-nitratesols, where high nitrate means a sol which has a NO /Pu mole ratio inthe range 0.6 to l. High-nitrate sols have a limited utility for makingP1102 microspheres because of the deleteriously high nitrate content andhigh surface area of the solid phase of the sol which result inextensive cracking of calcined microspheres.

APEX sols have been found suitable for making PuO -UO microspheres frommixture of an APEX sol with a U0 sol. There are, however, some severeshortcomings. Mixed sols of APEX-derived PuO and U0 must be maintainedat about 4-5 C. in order to provide adequate shelf life for futureprocessing to microspheres. Moreover, while a high-anion (nitrate) solis satisfactory for shelf life stability of PuO sols, high anionconcentrations detract from the stability of mixed PuO -UO sols.

Static stability, as measured by shelf life, as well as operationalstability of sols, has been associated with crystallinity as opposed toamorphous or polymeric solid phase of the sol, low nitrate (0.1-0.4) asopposed to high (0.6-1.0), and crystalline size. The crystallite size ofurania in urania sols is inherently much larger than plutoniacrystallites regardless of the sol preparation method, although thesemethods do not employ temperatures in excess of -60 C. The size ofurania crystallites can also be additionally increased by digestion ofthe sol in the temperature range -100" C. Plutonia sols in general andAPEX sols in particular consist of very small crystallites which show nodetectable change in crystalline size when treated by any known methodat temperatures below C. While it is possible to reduce the NO /Pu moleratio of APEX sols by exhaustive extraction of concentrated sols at lowtemperature, there is no detectable crystallite growth, and the nitrateloss is accompanied by random aggregation of crystallites which isgenerally undesirable. Results from thermal denitration of sols preparedby a precipitation process show that high-nitrate sols require extensivebaking periods at high denitration temperatures before an acceptably lowNO Pu ratio is reached; and, while higher denitration temperatures willreduce the baking time necessary to reach a low-nitrate sol (i.e., witha NO /Pu mole ratio of less than 0.2), very uniform heating is required.If overheating occurs, it is frequently impossible to redisperse thebaked product back to sol form.

While high-nitrate plutonia sols prepared by solvent extractionprocedures can be processed to gel microspheres, it has not beenpossible to obtain dense calcined microspheres with satisfactory yields.The principal problems involve particle malformation during thesphere-forming process, and a pronounced tendency for the particles tocrack and disintegrate during forming, drying, and calcining. The highnitrate content and high surface area of these plutonia sols also limitthe ability to mix them with thoria sols since unstable mixturesfrequently result. These problems associated with high-nitrate sols havebeen interpreted as resulting from high surface area and the highnitrate content which contribute to the physical and chemicalinstability of mixed sols and to the malformation experienced during thesphere formation process. Experienced investigators in this technologyhave long sought for a plutonia sol which is a low-nitrate sol and whichcontains large crystals of plutonia of the order of 50 A. or more asopposed to a crystal size of 10-20 A. on the reasoning that the largercrystals associate less nitrate due to the much lower surface area ofthe plutonia.

3 SUMMARY OF THE INVENTION With this background in mind, it is theprincipal object of this invention to provide a plutonia sol which has auseful shelf life and which can be subsequently processed to densemicrospheroidal particles.

It is another object of this invention to provide a lownitrate plutoniasol which has a large crystallite size.

It is still another object of this invention to provide a plutonia solwhich is both statically and operationally stable.

It is a further object of this invention to provide a method forrealizing the aforementioned objects.

In its process aspect, the present invention may be regarded as animprovement in the APEX process involving two essential steps in which(1) a portion of a previously prepared APEX sol is added as a seed to afeed solution of plutonium nitrate as it undergoes nitrate extraction toform a sol and where (2) the resultant seeded sol can be converted by abaking operation to a low-nitrate plutonia sol with large crystallitesize.

We have discovered that, when a plutonium nitrate solution is seededwith a plutonia sol as the solution undergoes nitrate extraction, adistinctive ordered aggregation of particles, micelles, occurs whichhave been found, by electron microscopy techniques, to be as large as150-200 A. and are statically stable. Moreover, this aggregated micellestructure is not affected by sol aging at ambient temperature, byextensive nitrate extraction, or by boiling at 100 C. That is to say, wefind that the seeded sol retains the sol form, but does not undergocrystallite growth. More specifically, seeded sols at this stage do notappear to afford any significant process advantage over regular APEXsols since both sols have comparable high NO /Pu mole ratios (0.6 to0.8), and mixed U PuO sol stability is not improved by forming theaggregated micelle structure only. In other words, mixed sols of PuO andU0 derived from solvent extraction processes still need to be cooled to4 C. to effect static as well as operational stability. We havediscovered, however, that the seeded sol with its aggregated micellestructure can be effectively denitrated to a low-nitrate sol and undergocrystallite growth when taken to dryness and heated. Specifically,low-nitrate sois (NO /Pu 0.1-0.4) with average crystallite diameters (asmeasured by X-ray line broadening or electron micrograph techniques) inthe range 30-80 A. can be prepared when a sol, with the appropriateaggregated micelle structure induced by seeding, is taken to dryness andheated for to 120 minutes at temperatures in the range 190-230 C. Whenthe seeded APEX sol is baked under thermal denitration conditions, weobtain a baked product which upon redispersion with water produces alow-nitrate sol which has crystallites in the range 30 to as high as 100microns. This is in direct contrast to the behavior of regular APEX solswhich show no detectable change in crystallite size or any significantreduction in the NO /Pu mole ratio when heated to these temperatures formuch longer times.

To form the product sol which is to serve as feed for microsphereformation, the baked product is mixed with water to effect redispersionof the solids into colloid form.

The seeding effect Several variations in the seeding technique can beused to obtain the desired aggregated micelle structure. In the broadsense, the development of the desired micelle structure can be effectedby introducing a plutonia sol during solvent extraction of a feedplutonium nitrate soluton. Alternatively, and for reasons of consistencyand reproducibility, we prefer to use a parent seed sol which Was itselfprepared by a seeding technique. This we call double seeding.

We can achieve a maximum micelle-forming ehect by multiple seeding bywhich we mean incremental addition of th tee ni rate so u on d r g o tnuous extraction in a single run. A multiple-seeding eifect is obtainedbecause extraction is continued after each feed addition until plutoniumpolymerization occurs-and hence a sol is formed before the next additionof feed. Single-seeding experiments have been found to produce a sol inwhich an average micelle size of 50-75 A. converts to an averagecrystallite size of 35-40 A. in diameter. By the multiple-seeding effectwe have obtained average crystallite sizes in the range 30 to a maximumof A. after baking the seeded sol and redispersion of the baked seededsol.

The presence of the desired aggregated micelle structure can be detectedby electron micrography and electron diffraction. In studies ofnon-seeded as well as seeded sols, we have established the necessity ofseeding as the means for producing micelle structure in plutonia sols;and, by studying sols produced by redispersion of dried and baked sols,seeded or unseeded, we have established the relationship between seededsols as the requisite source for lownitrate and large-crystalliteplutonia sols.

The amount of seed determines the existence and amount of micellestructure. A minimum of 20 percent of the total plutonium (seed-l-feed)for single seeding and 25-30 percent for double seeding is required foreffective development of the aggregated micelle structure. The NO /Pumole ratio of the seed can be varied over fairly wide limits from 0.7 to1.2. The age of the seed appears significant, provided only that itremains as sol and is not gelled. The seed polymer can be added to thefeed (plutonium nitrate) solution prior to or during extraction,provided the conductivity of the feed is no less than about 70millimhos/cm. Poor results in terms of incomplete aggregation areobtained when seed is added to a feed having lower conductivity.

The concentration of plutonium in the aqueous feed solution can bevaried over wide limits, from 15 up to 30 g. Pu/l. of solution. However,at a plutonium concentration over 30 g./l., excessive plutoniumextraction will occur using present techniques and the nitric acidconcentration may range from 0.5 to 1.0 M. A concentration of 26 to 28g. Pu/L; 0.6 M HNO provides feed with an initial conductivity of about100 millimhos/cm.

Seeding (single, double, or multiple) may begin immediately uponAPEX-type extraction of such solutions and continued until a NO Pu moleratio in the range 0.7 to 0.9 is attained. Higher values appear todecrease the thermal denitration rate and lower values result inplutonia sols which will not resuspend after baking. APEX and similarsolvent extraction processes are capable of reducing the NO /Pu to aminimum of --0.6 even with a digestion step to produce a sol with goodshelf-life stability. Further reductions in nitrate content by solventextraction cause gelling or precipitation.

The aggregated micelle structure resulting from seeding promotes rapiddenitration during baking. For example, a sol containing the desiredaggregated micelle structure prepared by double seeding will denitrateto a NO /Pu mole ratio of about 0.15-0.17 in 10 minutes at a bakingtemperature of 230 C. and will denitrate additionally to a value as lowas 0.1 without adversely affecting the final sol in 30-40 minutes. Forsols of this type, the micelles convert to an average crystallite sizeof about 60 A., and conversion to the larger crystallites appears to beessentially complete after 10-15 minutes of baking time. By comparison,an unseeded APEX sol with the same initial NO /Pu ratio will still be0.4 after 4 hours of baking at 230 C.

A distinctively advantageous feature of this invention is that the driedsols are quite insensitive to overheating. The dried sols can be bakedfor extended periods even after reaching a prescribed NO' /Pu conditionWithout adversely affecting the characteristics of the sol formed byredispersion. On the other hand, unseeded plutonia sols (which do nothave the aggregated micellular structure) produced from solventextraction or precipitation processes are exceedingly sen itive o bakingtime, leading to unpredictable final NO /Pu ratio and even to bakedproducts which will not resuspend to sol.

In order to reach a desired low-nitrate plutonia sol with largecrystallite size, we employed a baking temperature in the range 180-240C. and preferably in the range 200230 C. to produce a baked productwhich is easily redispersable to the final stable product sol. In atypical cycle, the APEX sol or other sol derived from solvent extractionis evaporated to dryness in air, rapidly heated to the desired bakingtemperature, and then held at the selected baking temperature for aperiod of to 60 minutes. The baked product is allowed to cool to roomtemperature and then redispersed to form the desired sol.

Having described the invention in general terms, we now provide specificembodiments which illustrate in specific examples how the invention maybe practiced to produce plutonia and mixed plutonia-urania sols ofenhanced stability.

EXAMPLE I Preparation of the seed sol To prepare seed material, 500 ml.of plutonium nitrate feed solution (Pu=26 to 28 g./l. HNO =0.60.8 M) wascontacted with about 3 liters of n-hexanol in the APEX extractionequipment. The initial conductivity of the feed solution was 100millimhos/cm. The feed solution was extracted continuously at 25 C.until the conductivity was reduced to 6 millimhos/cm. which correspondsto a NOg/PH mole ratio of 0.8i0.1. The sol was removed from theextraction equipment and concentrated by evaporation to 150-200 ml. Thisconcentration step is made only to provide a convenient volume forsubsequent handling and does not affect the sols properties.

EXAMPLE II Preparation of single-seed sol Five hundred ml. of plutoniumnitrate feed (Pu=26 28 g./l.; HNO =0.6-0.8 M) was charged to theextraction equipment and contacted with 3 liters of n-hexanol at 25 C.until the conductivity of the feed solution was reduced to 80 millimhos/cm. corresponding to a NO Pu ratio of -7.0. Forty-four ml. of the seedsol was added (Pu=74.9 mg./ml.) to the feed without interruptingextraction. This provides a plutonium seed concentration which is 20percent of the total plutonium in the system. The extraction wascontinued at 25 C. until the conductivity of the sol was 6 millimhos/cm.corresponding to a NO /Pu mole ratio of 0.8:01 The sol was removed rornthe extraction vessel and concentrated to a convenient volume (-O ml.)by evaporation.

EXAMPLE III Preparation of double-seed sol Five hundred ml. of plutoniumnitrate feed solution 6 the extraction equipment and contacted with 3liters of n-hexanol at 25 C. until the conductivity of the feed solutionwas reduced to millimhos/cm. Seventy-nine ml. of single-seed sol(Pu=72.5 rug/m1.) was added to the feed solution. This provides aplutonium seed concentration which is 30 percent of the total plutoniumin the system. The extraction was continued until the conductivity ofthe sol was 6 millimhos/cm. (NO /Pu mole ratio 0.8:0.1). The sol wasthen evaporated to dryness and baked. The sol was taken to dryness at80-90 C. and the temperature was not allowed to exceed C. The dry masswas then charged to a baking apparatus consisting of a stainless steelpan in an aluminum block containing a heating element. The bakingapparatus was preheated to 230 C. before the solids were added. Duringbaking, a lid was placed over the pan and air'was pulled through thebaker to remove gaseous decomposition products. Baking times of 15, 40,and 75 minutes at temperature resulted in NO /Pu mole ratios of 0.16,0.13, and 0.10, respectively, where the NO Pu concentrations weremeasured in the sol resulting from dispersion of the baked mass. The solcan be heated for at least an additional 60 minutes without reducing theNO /Pu mole ratio or otherwise noticeably affecting the solscharacteristics. The solids are then removed from the baking ap paratusand suspended in water. Excess water can be used if desired and the solcan be concentrated to greater than one molar by evaporation.

EXAMPLE IV Single-run/multiple-seeding sol preparation Fifty ml. of seedsol (116 mg. Pu/ml.) was mixed with 50 ml. of plutonium nitrate feed(Pu=21.5 nig./ml.; HNO =0.6 M) and extracted with 3 liters of n-hexanolat 25 C. until the conductivity was reduced to 21.6 millimhos/cm. Onehundred ml. of feed was added which increased the conductivity to 90millinihos/cm. and extraction was continued until the conductivity wasreduced to 22.5 millimhos/cm. One-hundred-fifty ml. of feed was addedand extraction was continued until the conductivity was 25.5millimhos/cm. Two hundred ml. of feed was added and extraction wascontinued until a final conductivity of 6 millimhos/cm. was obtained.The extraction was continuous, and the total extraction time was about30 percent less than that required in the experiments described above.The sol was then removed from the extraction equipment, evaporated todryness, and baked as previously described. The final sol wasessentially identical to sol obtained by double seeding.

The sols formed in accordance with the preceding examples produce alow-nitrate sol derived from the baking procedure with enhancedstability and larger crystallite sizes than have been possible by othermethods of which we are aware. The effect of drying temperature onseeded (Pu=2628 g./l.; HNO =0.60.8 M) was charged to 55 as well asunseeded sols is shown in the following table. TABLE-COMPARISON OFPLUTONIA soLs PREPARED BY SEVERAL MODIFICATIONS OF THE APEX PROCESSApproximate average Approxi- Nos/Pu crystalline mate mole ratio size ofP1102 Mixed UOg-PUO z drying of unmixed in P1101 sol sol shelf life Solpreparation C.) P1102 501 (A.) at 25 0.

Regular APEX 0. 71 10-20 30 min.

Do... 200 0.63 10-20 1lir.45 rnin. D 0 230 0. 68 10-20 1 hr. 45 min.Continuous extraction APEX 0.67 10-20 1 hr. 50 min.

v Do 2 200 0. 68 10-20 10 hr. Single seeding continuous extraction- 1800. 53 20 hr.

Do 230 0. 39 30-40 132 hi. (5% days). Double seeding continuousextraction 0. 64 10-20 1 hr.

Do 0. 63 2 hr. 30 min. Do 0. 50 10 hi. Do. 200 0. 41 48 hr. Do 220 0. 2055-65 138 hr. Multiple feed addition. 230 0.19 55-75 192111. (8 days).

1 Not dried.

2 Denotes continuous extraction of plutonium nitrate solution withn-hexanol down to a NOa/Pll mole ratio of -10 without digestion at 100 C3 Consists generally of both aggregates of small crystallites (IO-20 A.)and 30-40 A. crystallites which resulted from conversion of aggregates.

4 Consists of both aggregates of small (10-20 A.) and 65-75 A.crystallites.

As seen from the table, drying temperatures in the range 160230 C. wereused. When sols were dried at the higher temperatures, they wereevaporated to dryness slowly at temperatures below 100 C. The dry solidswere then rapidly heated to the desired temperature, held at thistemperature for 30 minutes, removed from the heat, and allowed to cool.Sols which were not dried were concentrated to -1 M PuO by evaporationand extracted three times with n-hexanol. Ten volumes of n-hexanol werecontacted with one volume of sol during each extraction. This procedurewas expected to remove all of the extractable HNO Mixed sol stabilitytests were made by mixing sufficient P110 sol with CUSP-prepared U sol(which had been freshly contacted with an equal volume ofwater-saturated n-hexanol) to provide a total metal concentration of0.95 M (80% U0 20% PuO and the sols were aged at 25 C. until gelationoccurred.

Electron micrographs and electron dififraction patterns were obtainedfor all of the sols shown. There was no apparent change in crystallitesize or crystallinity for the unseeded sols when they were dried atZOO-230 C. The NO Pu mole ratios for unseeded sols were not affected bythis heating step, indicating that there was no appreciable change incrystallite size. Seeded sols showed a regular decrease in NO Pu moleratio with increasing drying temperature, with corresponding increasesin average crystallite size. This increase in size was apparent fromboth electron micrographs and electron diffraction patterns of the driedsol. The micrographs indicated that the increase in average crystallitesize with increasing drying temperature does not result from gradualgrowth of small crystallites, but from an increasing number of micelleaggregates which have converted to larger crystallites. For example, inthe double-seeding runs shown, a micrograph of the undried soilindicated that very small crystallites (100-20 A.) are primarilyaggregated to form micelles which vary in diameter from 50 A. to -180 A.Micrographs at the intermediate drying temperatures (160, 180, and 200C.) also showed micelle aggregates present. However, a change in theelectron diffraction patterns for material heated at 180 C. and 200 C.indicated that large crystallites are also present, correlating with thedecrease in NO /Pu mole ratios shown in the table. At 220 C., a ratherdramatic change in the appearance of the micrograph occurred because themicelles had been completely converted to larger crystallites. Thiscomplete conversion was also indicated by the electron diffractionpattern and the NO Pu mole ratio.

These changes in sol characteristics are also reflected by theimprovements in mixed UO PuO sol shelf life at 25 C., as shown in theright-hand column of the table. The data reflect that a substantialincrease in shelf life is obtained even before complete conversion ofthe micelles to crystallites occur; that is, with sols baked in the180-200 C. range.

One very important feature of obtaining relatively large crystallites bythe seeding-and-baking technique is that the dried sols appear to bequite insensitive to overheating. We have found, for example, thatseeded sols could be heated much longer than necessary for optimumcrystallite formation without adversely alfecting final solcharacteristics.

In summary, we have shown that low-nitrate sols can be prepared (NOPu=O.l0.2) with average crystallite diameters of -60 A. when a seededsol with the appropriate aggregate structure is taken to dryness andheated for to 60 minutes at a temperature in the preferred range ofZOO-230 C. While lower temperatures may be used down to 190 C., thedenitration times become excessively long. This is in direct contrast tothe behavior of unseeded APEX sols, which show no detectable change incrystallite size or any significant reduction in the NO Pu mole ratiowhen heated at these temperatures for much longer times. The speciallyprepared sols with low nitrate-tolutonium mole ratios an with largercrystallites xh b greatly improved mixed-sol shelf life. For example,the time to gelation for 0.9-1.0 M sol UO -2O% P110 at 25 C. isincreased from 1 hour for regular APEX sol to 5-8 days for the modified(seeded, baked, and redispersed) products. We have also formed pureplutonia microspheres which calcine to dense, crack-free products. Thishas not been possible with high-nitrate plutonia sols to a successfuldegree, including regular APEX sols, because of extensive cracking anddisintegration of the spheres during calcination. Excellent results werealso obtained in small-scale experiments when these larger crystallitesols were mixed with CUSP sols and formed into microspheres. StandardCUSP-APEX forming conditions were used in these experiments except thatit was not necessary to cool the mixed sols so they were handledentirely at room temperature.

EXAMPLE V Preparation of mixed 80% UO 20% Pu0 microspheres Twenty ml. ofCUSP urania sol (U=238 g./l., NO /U=0.11, HCOO/U=0.45)

was contacted with 20 ml. of Water-saturated n-hexanol. The sol wasseparated from the alcohol phase and mixed with 5.3 ml. of low-nitrateAPEX sol (Pu=225 g./l., NO /Pu=0.l9) at room temperature (-25 C.) Twelveml. of the mixed sol was fed to a microsphere-forming column after thesols were mixed and the remainder of the sol was formed into spheres 4hours later to stimulate long-term operations. The drying medium in thecolumn was 2-ethyl-l-hexanol which contained 0.15 vol. percent Span-80(a surface-active agent comprising sorbitan monooleate) and 0.40 vol.percent Pluronic L-92 (a nonionic surfactant obtainable from WyandotteChemical Corporation, Wyandotte, Mich.). The set gel spheres wereremoved from the column and dried in an argon gas stream at roomtemperature. The spheres were then heated slowly (SO-100 C.rise perhour) to 1150 C. in a 4% H -Ar atmosphere and allowed to cool. The twobatches were dried and calcined separately. Dense, crackfreemicrospheres 400-600 in diameter were obtained in both cases and productyields were 95% for both batches.

A CUSP sol refers to the method of forming urania sols as described inUS. patent application Ser. No. 814,311 of common assignee, filed Apr.8, 1969, where a predominantly crystalline urania sol is prepared froman acid-deficient solution of uranous nitrate by heating the solution toa crystallizing temperature of between 58 C. and 65 C. and removingnitrate ions at that point at a rate which approximates the rate ofrelease of free acid to the aqueous phase of the resultant sol. CUSPsols are used by way of example. Other urania sols may also be used toproduce UO -PuO mixed sols and microspheres.

What is claimed is:

1. A method of preparing a plutonia sol which comprises mixing a seedplutonia sol with an aqueous solution of plutonium nitrate; extractingnitrate from the resulting mixture until a sol having a conductivity inthe range 5 to 7 millimhos corresponding to a NOg/Pll mole ratio in therange .7 to .9 is reached; drying the resulting sol to solid; heatingsaid solid to effect denitration and crystallite growth; and thendispersing the baked product to form a low-nitrate sol having an averagecrystallite size in the range 30 to angstroms.

2. The method according to claim 1 in which the amount of seed sol issufficient to produce an aggregated micellular structure resulting fromnitrate extraction of said plutonium nitrate solution.

3. The method according to claim 1 in which the seed sol is itselfderived from the nitrate extraction of a solution of plutonium nitrate.

4. The method according to claim 1 in which the dried sol is baked at atemperature in the range 190230 C. for a period of 10 to minutes.

5. A method for converting a plutonium nitrate solution to a sol whichcomprises contacting a first portion of said solution with an aqueousimmiscible organic alcohol or amine to selectively remove nitrate ion tothe point where that portion of solution is converted to sol, adding asecond portion of said solution to the sol produced from the firstportion with continuous nitrate extration until a sol is produced fromthat second portion, and continuing to add increments of solution to theaqueous phase in contact with nitrate extracting alcohol or amine untilthe entire solution has been converted to a final sol having aconductivity of no less than about 6 millimhos/cm.

6. The method according to claim 5 in which the final sol is dried andthen baked to effect denitration and crystallite growth and thendispersing the baked product to a product sol having a NO Pu mole ratioin the range 0.1 to 0.4 and an average crystallite size in the range 30to 100 angstroms.

7. A method for preparing a low-nitrate plutonia sol which comprisesextracting an aqueous solution of plutonium nitrate with an aqueousmiscible alcohol or amine to efiect selective extraction of nitrate ionsto the point where a plutonia sol occurs, introducing a seed plutoniasol into the aqueous phase before or during the nitrate extraction,drying the sol and then heating the dried sol to a temperature in therange l90230 C. to effect denitration and crystallite growth, and thendispersing the baked product to produce a low-nitrate so] with anaverage crystallite size in the range 30 to 100 angstroms.

References Cited UNITED STATES PATENTS 3,600,323 8/1971 Tallent 252301.l3,461,076 8/1969 Lloyd et al 25230l.l 3,513,101 5/1970 Meservey252--301.l 3,310,386 3/1967 Lloyd 23--344 CARL D. QUARFORTH, PrimaryExaminer R. L. TATE, Assistant Examiner U.S. C1. X.R. 423251

